WASHINGTON, August 9, 2010 (AFP) - Even modest rises in global temperatures will drive down rice production in Asia, the world's biggest grower of the cereal grain that millions of poor people depend on as a staple food, a study published Monday warned.

Researchers from the United States, the Philippines and the Rome-based Food and Agriculture Organization (FAO) looked at the impact of rising daily minimum and maximum temperatures on irrigated rice production between 1994-1999 in 227 fields in China, India, Indonesia, the Philippines, Thailand and Vietnam.

They found that the main culprit in cutting rice yields was higher daily minimum temperatures.

"As the daily minimum temperature increases, or as nights get hotter, rice yields drop," said Jarrod Welch of the University of California, San Diego, and lead author of the study published in the Proceedings of the National Academy of Sciences of the United States.

"Up to a point, higher daytime temperatures can increase rice yield but future yield losses caused by higher night-time temperatures will likely outweigh any such gains because temperatures are rising faster at night," Welch said.

Rising temperatures in the past 25 years have already cut rice yields at several key growing locations by 10-20 percent.

The loss in production is expected to get worse as temperatures rise further towards the middle of the century, said Welch.

"If daytime temperatures get too high, they also start to restrict rice yields, causing an additional loss in production," he said.

Rice is a key global crop, eaten by around three billion people a day. In Asia, it is a staple food to some 600 million people who are among the world's one billion poorest inhabitants, the study and FAO data show.

A decline in rice production will mean more people will slip into poverty and hunger, the authors of the study warned.

"If we cannot change our rice production methods or develop new rice strains that can withstand higher temperatures, there will be a loss in rice production over the next few decades as days and nights get hotter," said Welch.